BP23 Prediction of Summer Stratus in San Francisco BayAided by Low-Level HYSPLIT Trajectories

Gary P. Ellrod EWxC, LLC, Granby, CT

1. Introduction

Author’s Email Address: Gary.Ellrod@gmail.com Web site: www.ellrodweather.com

The prediction of stratus and fog in the San Francisco Bay region is challenging and important for anticipating conditions that may result in extensive delays for inbound and departing air traffic. San Francisco International Airport (SFO) for example, is typically in the top ten among airports in the United States with the longest average arrival and departure delays, almost solely due to stratus. The closely spaced runways at SFO require visual contact for inbound aircraft, which cannot occur with low clouds present, requiring greater spacing and increased delays. (Reynolds et al. 2012)

A critical element in these forecasts is low level flow from the Pacific Ocean into the Bay Area. Trajectories allow forecasters to visualize the origin of the air mass over the area. The National Oceanic and Atmospheric Administration (NOAA) Air Resources Laboratory's (ARL) Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model can provide forecasts of low level trajectories using data from a variety of numerical prediction models generated by the National Centers for Environmental Prediction (NCEP). These trajectories can help forecasters visualize the inflow of marine air into the Bay region.

7. Findings

In the San Francisco Bay area, mean 18h HYSPLIT trajectories from 1800 UTC show promise in predicting stratus for the following morning at SFO or OAK. A preliminary forecast system was developed to show the likelihood of stratus within three sectors using the combined 6h forecast points. Verification during summer 2015 indicated that the sector method for stratus prediction works best at OAK, and results for no stratus (SSE sector) verify best at SFO.

8. ReferencesDraxler, R.R., and G.D. Hess, 1998: An overview of the HYSPLIT_4 modeling system of trajectories, dispersion, and deposition. Aust. Meteor. Mag., 47, 295-308. (Available at: http://www.arl.noaa.gov/documents/reports/MetMag.pdf

Reynolds, D. W., D. A. Clark, F. W. Wilson, and L. Cook, 2012: Forecast-based decision support for San Francisco International Airport. Bull. Amer. Meteor. Soc., 93, 1503-1518.

4. Mean Trajectories

Mean 18h HYSPLIT trajectories for 37 cases of stratus at both SFO and OAK and 30 cases of no-stratus were obtained in May-Sep of 2013-14, for a starting point west of SFO (37.5N -123W). Mean vectors below (red lines) show pronounced onshore flow for stratus days, but stronger along-shore flow on cloud-free days. Thus, HYSPLIT may help with afternoon forecasts of stratus at SFO/OAK for the following morning.

2. Procedures

Sample dialog boxes for running 6x6 matrix of HYSPLIT trajectories along the South Atlantic Coast (see panel 5) using 2-deg grid spacing. HYSPLIT runs in <1min on a PC.

HYSPLIT18h trajectories from an initial time of 1800 UTC were obtained for cases of non-frontal stratus and no stratus for San Francisco Bay, based on aviation observations at SFO and Oakland (OAK) during the summers of 2013-14. The trajectories used data from the Global Data Assimilation System (GDAS) at either 1.0 deg or 0.5 deg resolution. Starting point of the trajectories was at 37.5N 123W at a height of 50m. The dialog boxes below show the procedures for generating a 6 x 6 matrix of trajectories.

Website URL for NOAA HYSPLIT model is: http://ready.arl.noaa.gov/HYSPLIT.php

5. Forecasting Approach 6. Verification

3. Sample CasesIn Northern California coastal stratus events, HYSPLIT trajectories typically show pronounced flow into coastal bays such as San Francisco and Monterey. Days with no fog showed predominately alongshore trajectories toward Monterey or points west. The onshore flow is forced by the presence of a heat low in the interior Central Valley (lower right), which leads to a strong pressure gradient from the coast to the interior. Sea surface temperatures along the coast are very cool (top right) due to upwelling caused by the low level northerly jet, leading to condensation in the boundary layer.

NOAA AVHRR SST, 1 month mean centered on 1 Jun 2013 (NOAA CoastWatch)

18h HYSPLIT forecast from 18Z, 29 Jun 14 (left), and GOES-15 visible image at 14Z, 30 Jun ‘14 (right)

18h HYSPLIT forecast from 18Z, 8 Jul 14 (left), and GOES-15 visible image at 14Z, 9 Jul 14 (right)

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Mean 18h HYSPLIT trajectories for 30 cases of stratus (yellow) and no stratus (white) at SFO and OAK

Using the combined 6h forecast HYSPLIT points for stratus and no-stratus cases (red dots above) at both SFO and OAK, the likelihood of stratus at both SFO and OAK the following morning was estimated by calculating the percentage of stratus points within three sectors (NE, ESE, and SSE). Most of the points in the NE sector (which includes SFO/OAK and approach zones) were stratus events, while no stratus occurred in the SSE sector.

HYSPLIT 6h forecast positions were obtained each day during the summer (Jun-Aug) of 2015 and verified using aviation observations at SFO and OAK. The NE and ESE sector locations for stratus prediction (N=48, 31) (see panel 5) verified best at OAK (blue boxes), while the no-stratus SSE sector forecast (N=14) worked well at SFO. A correct stratus prediction required ceilings observed for at least two consecutive hours between 1200 and 1800 UTC (5AM – 11AM PDT) the following morning.

NCEP Surface analysis for stratus case (L) and no stratus case (R)